Geroller hydraulic motors receive pressurized fluid as an input and provide high torque rotational movement as an output. The gear sets of the hydraulic motor drive assembly cooperatively define fluid chambers. The chambers expand when hydraulically connected to a source of pressurized fluid and contract when connected to a drain that returns the fluid to the source. The expansion and contraction of the fluid chambers causes the rotational movement.
These motors are relatively small and efficient, and the rotational output is widely used to move and control various types of equipment. In many of these uses, a human operator controls a source of pressurized fluid for the hydraulic motor. This controls the input to the hydraulic motor and, in turn, controls the rotational output (speed and torque) of the hydraulic motor.
Geroller hydraulic motors can exhibit cogging at relatively low speeds. Cogging is a jerking or detenting or variation in the rotational output speed of the hydraulic motor that (a) occurs during each complete (360 degree) rotation of the motor output, (b) at a frequency measured in cogs per revolution that is related to the number of teeth in the geroller gear set in the hydraulic motor drive assembly, and (c) is accompanied by measurable pressure variations in the input to the hydraulic motor. All geroller hydraulic motors may tend to exhibit some amount of cogging at low operating speeds as a gear in one of the gear sets rotates into mating alignment with a gear in the other gear set and hydraulic fluid passages connected to the fluid chambers are opened and closed. Cogging can result from dimensional tolerances in the hydraulic motor.
Even if a hydraulic motor exhibits cogging, the cogging may not be objectionable in some equipment in which the hydraulic motor is used under some operating conditions. For example, if a hydraulic motor is used to rotate brushes in an automatic car wash system, the brush may rotate at a relatively constant speed when the equipment is operated and precise control at low speeds during starting and stopping may not be needed. In this instance, cogging at low speeds might not be objectionable.
In certain operating conditions of other types of equipment in which geroller hydraulic motors are used, however, cogging can be objectionable. Objectionable means that an ordinary human operator of the equipment that is experienced in operating the equipment would (a) notice the cogging under specific operating conditions, and (b) prefer that the cogging be eliminated in order to improve performance of the hydraulic motor and of the equipment in which the hydraulic motor is used under those operating conditions.
As an example of an application for geroller hydraulic motors in which cogging at low speed can be objectionable, geroller hydraulic motors are frequently used in lawn equipment including lawn mowers to control the equipment's drive wheels. The drive wheels are rotated by the hydraulic motor to propel the vehicle. In that use, a variable displacement hydraulic pump can be used to provide the pressurized fluid input to control the geroller hydraulic motor. One pump and one hydraulic motor can be associated with each of the drive wheels of the equipment. The human operator can use control levers that separately control the output displacement of each of the variable displacement pumps, so that the rotational speed and rotational direction of each hydraulic motor, and the rotational speed and rotational direction of each drive wheel rotated by that motor, is controlled. Because each pump and motor associated with each drive wheel is separately controlled, the human operator can precisely control forward and reverse speed and turning of the equipment.
In many operating conditions for this type of equipment, the equipment is operated at or near maximum over the ground speed. In these operating conditions, the hydraulic motors are operated at or close to their maximum rotational speed as determined by the maximum hydraulic fluid displacement of the variable displacement hydraulic pump associated with each motor. Under these conditions, any cogging of the geroller hydraulic motors may not be objectionable. In other operating conditions, however, the hydraulic motors are used at low rotational speeds and at high fluid pressures. Low rotational speed of a geroller hydraulic motor means less than five revolutions per minute of the output shaft of the motor, and high pressure means greater than 1000 pounds per square inch fluid pressure at the inlet port of the motor. These operating conditions can include propelling the equipment up a sloped surface at relatively slow speed, such as propelling the equipment up a sloped lawn surface or up a ramp for loading the equipment into a trailer or a truck. Under these operating conditions, cogging of the hydraulic motors can be objectionable. Objectionable cogging can also occur in other types of equipment and in other types of hydraulic circuits that incorporate geroller hydraulic motors and in other conditions.